1. Technical Field
The present invention relates to the general field of patterning by means of lithography. More particularly, the present invention relates to the thermal distortion of the photomask caused by substrate heating during electron beam patterning and to procedures, apparatus and materials for reducing heat-induced photomask distortion, leading to increased patterning accuracy.
2. Description of Related Art
The production of precise patterns on surfaces is a necessary stage in the fabrication of integrated circuits, and finds applicability in many other commercial environments as well. The typical method for creating such patterns is to coat the surface to be patterned with a chemical that undergoes a chemical transformation upon exposure to energy, a “resist.” Positive resists undergo chemical transformation on exposure to energy leading to removal of resist from the surface to be etched in the regions so exposed. Negative resists undergo other chemical transformations, such as cross-linking, leading to removal of resist in regions not exposed to energy. Both positive and negative resists are commercially useful. Thus, exposing a resist-coated surface to the appropriate pattern of energy leads to selective removal of resist according to that pattern (exposed or masked), uncovering selected regions of the underlying surface to further chemical etching in a subsequent etching step. Removal of all resist following surface etching leads to the desired pattern etched into the surface.
The energy incident on the resist is typically either electromagnetic or a beam of particles, typically ions or electrons (“e-beam”). In addition, the energy may be directed onto the resist in one of two general ways: 1) through a mask having both transparent and opaque regions therein permitting selective passage of the incident energy to create the desired pattern of exposure on the underlying resist, or 2) as a focused beam, guided so as to impact selectively only those areas requiring exposure. Exposure through a mask is the presently preferred technique for producing numerous identical patterns at reduced costs. However, the mask itself must first be made, most commonly, by focused beam impact. Thus, focused beam exposure of resists remains a necessary step in the production of masks for lithography.
Direct beam “writing” of patterns onto resists has several advantages over use of a mask. Among these are avoiding the complications of alignment and registration of the mask and more precise patterning accomplished by precisely focused beams. Thus, beam lithography finds applicability in many areas of technology in addition to mask creation. However, the discussion herein will be particularly directed to e-beam lithography for the production of masks (also called photomasks) although other applications for the methods described herein will be apparent to those having ordinary skills in the art. For economy of language we will describe e-beam lithography as typically used in the manufacture of masks, not intending thereby to limit the scope of the invention.
In order to create even more precise patterns, it is necessary to identify each source of patterning error and strive to reduce all sources of errors. In particular, one source of error in patterning results from the heating of the substrate on which the photomask is being patterned, leading to distortion of the mask, positioning error and imprecise patterning. Theoretical and computational studies of such thermal distortions have been reported by Shamoun et. al. “Assessment of Thermal Loading-Induced Distortions in Optical Photomasks due to E-Beam Multipass Patterning” appearing in J. Vac. Science and Technology B, Vol. 16, No. 6, pp. 3558-3562 (1998), and also “Photornask In-Plane Distortion Induced During E-Beam Patterning,” appearing in Proceedings of the 1998 SPIE Symposium on Emerging Lithographic Technology II, Vol. 3331, pp. 275-279 (1998). Similar results related to thermal distortion during patterning have been reported by Groves, “Theory of Beam-Induced Substrate Heating,” appearing in J. Vac. Science and Technology B, Vol. 14, No. 6, pp. 3839-3844 (1996).
E-beam patterning typically directs a focused beam of electrons onto a target to write the pattern in a layer of photoresist. A typical mask structure is depicted schematically (but not to scale) as 1 in FIGS. 1 and 3. The top most layer, 14 in FIG. 3, is typically the resist upon which the beam 6 writes the desired pattern. Below the photoresist 14 is typically the mask material to be patterned 13. The photomask is typically a thin layer of chromium-containing material. A substrate 10 is typically located beneath the photomask, supporting the photomask and resist layers and into which a substantial portion of the beam's energy is deposited. The substrate is typically a glass or glassy-like material. The present invention relates to the deleterious effects on the patterning process caused by energy deposited in the substrate following passage of the beam through the resist and photomask. For economy of language, we will refer to the entire structure 1 as the “mask” or “photomask” intending to encompass thereby all layers including resist 14, mask-forming material 13, and substrate 10.
To be concrete in our description, we will describe the technologically and commercially important process of electron beam (“e-beam”) patterning of photomasks, as typically would be used for the fabrication of integrated circuits. However, the methods, apparatus and materials described herein for reducing thermal distortion during patterning may be used in other fields of application as apparent to those having ordinary skills in the art, and the present invention is not inherently limited to e-beam patterning of photomasks.
The thermal distortions of the substrate due to e-beam patterning are found to be a function of the system thermal boundary conditions, that is the thermal radiation absorbing/emitting properties of materials near the substrate during patterning. The thermal boundary conditions determine the rate of heat exchange between the mask and its surroundings. Since e-beam patterning is performed in a vacuum, radiative heat transfer is the dominant mode of heat exchange between the substrate and its surroundings. The rate of heat dissipation from the mask should be increased to lower the average temperature rise of the mask during patterning. We note that a uniform temperature rise of the substrate and photomask, not resulting in photomask distortion, would not be a serious problem in precise patterning as this effect is readily corrected by magnification adjustment. However, the exchange of heat energy between the substrate and its surroundings invariably results in non-uniform substrate heating and photomask distortion. Thus, reduction of thermal effects on the substrate is the preferred strategy rather than attempted uniformization of thermal effects on the substrate which is typically much more difficult.
One approach to maintaining accurate writing in the face of thermal distortions of the substrate is to calculate (or at least estimate) in real-time as the pattern is being written, the extent of thermal distortions. One then may correct the e-beam writing process to compensate for these thermal distortions. Estimation/correction is the approach taken by Veneklausen et. al. in U.S. Pat. No. 5,847,959. The present work is complimentary in that it relates to ways to minimize thermal distortion that one may additionally wish to estimate and correct. Estimation/correction in real-time becomes a more effective procedure if the corrections are as small as feasible.
Heat transfer out of the substrate is dominated by radiation, as expected for a substrate in a vacuum. The amount of heat transferred is found to be strongly dependent on the radiative properties (absorptivity/emissivity) of the material surrounding the substrate in the chamber. Although the heat deposited in the substrate is deposited pulse by pulse during e-beam patterning, and therefore has a structure and geometry within the substrate, such geometric effects are not strongly related to the overall thermal distortion of the substrate. Local deposition of heat energy (that is, pattern-dependent deposition of heat energy) rapidly dissipates (over a matter of seconds) to lead to the substantially global heating effects that are the subject of the present invention. Reduction of thermal distortion of the substrate during patterning by suppressing of radiation from the surroundings to the substrate is the subject of the present invention.